Did you think that Facebook’s amazing mobile ad story — mobile was a big, troublesome zero two years ago, and 62 percent of the company’s ad revenue yesterday — was fueled by app install ads?

Sheryl Sandberg wants you to know that this isn’t the case. Or, at least, that Facebook isn’t dependent on app install ads, which are just what they sound like — ads that try to get phone users to install apps like Candy Crush.

Here, via Seeking Alpha, is what Sandberg said yesterday during Facebook’s earnings call, when an analyst asked about the size of her app install business:

“I do think sometimes people think that mobile app install ads are all of the revenue or a great majority of revenue and they’re not. They’re only part of the mobile ads revenue. Our mobile ads revenue is pretty … it’s broad based. We have large brand advertisers, small SMBs, direct response advertisers as well as developers using our mobile ads. The mobile app install ads, which are run not only by developers but also by large companies that want to get people to install apps are growing. They remain a good part of our mobile ads revenue and we’re excited about the opportunities there. But we see our opportunities in mobile ads as much broader than just installing apps.”

See? It’s big. But not that big.

But note that the only qualifier Sandberg offers is that app ads don’t represent all or the “great majority” of Facebook’s revenue. Which could still mean they are very, very big.

In the past, I’ve heard industry people guesstimate that app ads account for half or more of Facebook’s mobile ad revenue. If that were true, that would mean app ads generated some $800 million for Facebook in the last quarter alone. J.P. Morgan’s Doug Anmuth thinks they now represent 20 percent to 25 percent of mobile revenue — something in the $400 million range. Even that makes them a billion dollar-plus business on an annual basis.

Whatever the number is, it’s big, and the envy of everyone in the ad world. That’s why Twitter, Google and Yahoo are all trying to catch up. So why wouldn’t Sandberg want to boast about it?

Two theories, which aren’t mutually exclusive:

Facebook isn’t sure that app ads, which showed up out of nowhere, are going to stick around. Developers are buying app ads because they work. But now that lots of people are using the ads, it’s not nearly as cheap and easy to acquire app users this way. And there’s also something about app ads that triggers flashbacks to AOL in the boom years, and Facebook doesn’t want any part of that.

App ads don’t fit Facebook’s larger narrative.. Facebook’s goal is ultimately to take big chunks of money that brand advertisers are spending on TV. There are some brands spending money on app ads. But they’re not the ones driving that business, because people don’t use brands’ apps (besides Starbucks, do you have any big brands’ apps on your phone?). So Candy Crush money is great, but it’s not GM and P&G and Coke money, and that’s what Sandberg really wants.

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